1. Field of the Invention
The invention relates to a power line communication system for performing communication between a plurality of devices by using a power line as a signal transmission channel, and to a power line branching apparatus for use in this power line communication system.
2. Description of the Related Art
In recent years, information communication at home is growing in need for the purposes of sharing peripheral equipment of computers, sharing information including documents, still pictures and moving pictures, and games, the Internet, and so on. Communication networks are thus in demand not only at offices but also at ordinary households.
These days, for communication technology to be used in constructing a communication network at home, power line communication technology is viewed as promising and is being developed. The power line communication technology utilizes a power line as a signal transmission channel. This technology makes it possible to perform communication between a plurality of communication devices by connecting each of those devices to a receptacle installed in each room in a house, for example. On the other hand, high speed communication technology at a high frequency band has been developed recently, and it is desired that a home communication network employing this high speed communication technology and the power line communication technology be put to practical use.
However, there arises a problem resulting from the fact that various types of electrical equipment is connected to an indoor power line that is to serve as the communication channel in power line communication. Specifically, the equipment connected to the indoor power line can generate noise or lowers an impedance of the power line, which can result in an increase in the error rate to cause communication failures in the power line communication.
Power line communication is typically performed by using normal mode signals. Therefore, in the power line communication, communication failures may occur from normal mode noise in particular among the noises generated by the electrical equipment.
It should also be noted that not every indoor power line is laid in an environment suitable for communication, because indoor power lines are not initially intended for use as communication channels. For example, impedance of indoor power lines varies due to the electrical equipment connected to the indoor power line, as mentioned above. Depending on the value of impedance of the indoor power line, signals may be reflected at connection points between the equipment for performing power line communication and the indoor power line, which results in signal deterioration.
In situations where no electrical equipment is connected to a connecting part for connecting electrical equipment to an indoor power line, such as receptacles, the connecting part makes an open termination, at which signals are reflected to thereby cause signal deterioration.
Published Unexamined Japanese Patent Application (KOKAI) 2001-285150 discloses a power line communication system for performing communication between a plurality of devices using a power line, in which a signal interrupting part is provided between an indoor power line and electrical equipment for blocking passage of signals for communication.
On the other hand, Published Unexamined Japanese Patent Application (KOKAI) 2002-217797 discloses a power line communication system in which an impedance matching circuit is provided between an indoor power line and communication equipment/electrical equipment, and an inductor for blocking passage of signals and noises is also provided between the indoor power line and the electrical equipment.
In a power line communication system, if no measure is taken against the impedance- or noise-related problems described above, communication failures can occur due to impedance variations and noises on the indoor power line.
The technique disclosed in Published Unexamined Japanese Patent Application (KOKAI) 2001-285150 is effective to reduce noise generated by electrical equipment. With this technique, however, it is impossible to set the impedance of the indoor power line to a desired value suitable for communication. Furthermore, even if this technique is employed, in situations where no electrical equipment is connected to the connecting part for connecting electrical equipment to the indoor power line, the connecting part makes an open termination at which signals are reflected. Thus, the technique disclosed in the above-mentioned publication document cannot eliminate the problem of signal deterioration.
The technique disclosed in Published Unexamined Japanese Patent Application (KOKAI) 2002-217797 makes it possible to set the impedance of the indoor power line to a desired value and to reduce noise generated by the electrical equipment. This technique, however, requires an impedance matching circuit to be located between the indoor power line and the communication equipment. The impedance matching circuit includes a resistor located between two conductive lines. Due to the impedance matching circuit located between the indoor power line and the communication equipment, the electric power of signals is consumed by the resistor and this makes the signals extremely weak.